While spring-loaded poppet-type relief valves are widely used in a variety of hydraulic circuits, they frequently suffer from unstable response and regulation problems. Generally, these poppet relief valves include a spool with a conical head, and upon reaching a predetermined opening pressure, the spool moves to admit a cone of fluid through an annular orifice. Since this orifice necessarily increases in size with higher flow rates, the loading spring must be further compressed and complex fluid impingement and flow forces are exerted on the head of the poppet relief valve which create stability and regulation problems.
In connection with stability problems, it is apparent that a poppet relief valve should rapidly respond to various operating parameters with a minimum of fluctuation. Unfortunately, relatively erratic operation thereof is often observed at a certain flow rate. For example, rapid oscillation of these poppet valves between a fully open position and a nearly closed position is sometimes so severe that the resultant flow and pressure fluctuations are frequently audibly evident. At other times, while it cannot be heard, the extremely fast pressure changes are still felt by the associated circuit with an accompanying deleterious effect thereon.
These poppet relief valves should also exhibit a relatively flat, controlled pressure versus flow rate characteristic. This is referred to as pressure regulation and is defined as the variation in pressure over the full flow range expressed as a percentage of the pressure at maximum flow, or as follows: ##EQU1##
Heretofore, 15% regulation was generally considered acceptable. But this undesirably high percentage was actually acceptable only as a compromise, since it was known that with a lower percentage of regulation the likelihood of significant stability problems greatly increases.
Reference is made to U.S. Pat. No. 3,199,532 issued Aug. 10, 1965 to R. E. Trick for a poppet relief valve constructed to overcome at least a portion of the aforementioned problems. It does this by closely controlling the diametrical clearance between an annular collar on the poppet valve head and the bore in which it is received, and also by controlling the axial position of the collar with respect to an annular outlet groove to provide an auxiliary, velocity compensating orifice in series with the primary regulating orifice, and a secondary pressure within an entry chamber which imposes loads on the poppet valve. It is apparent that the precise dimensional control required during the manufacture of both the valve head and its associated housing is very expensive. Moreover, even with this added complexity, the poppet relief valve fo U.S. Pat. No. 3,199,532 is not believed to have both the combination of good stability and a regulation level below 10%. This combination is particularly hard to achieve when these poppet relief valves are used in high pressure circuits, for example, above 2000 psi (140 Kg per sq. cm.). Still further, these problems are compounded when they are expected to handle a relatively broad flow range, for example, from zero to 130 gpm (0 to 8.2 liters per sec.).